The present invention is directed to an engine idle speed controller.
First, engine control during normal (that is, non-idling operations) will be described.
An engine control apparatus (hereinafter referred to as an engine torque demand system (ETD system)) has been devised to calculate a required target torque, in accordance with the driver's accelerator operation, external loads, and the like, and to perform control so as to cause the engine to generate the target torque.
Japanese Patent Publication 1-313636, for example, discloses an engine torque demand system designed to calculate an engine target torque in accordance with an, accelerator operation quantity, engine speed and an external load, and to control a fuel injection quantity and a supply air quantity in accordance with the target torque.
Such a torque demand type of engine control apparatus calculates a target generation torque by adding, to the required output torque (determined based on accelerator depression), a loss load torque such as friction torque appearing as a loss in the engine and power train system, and controls the fuel injection quantity and the supply air quantity so as to achieve the target generation torque
This torque demand system produces improvements in driveability by using, as a reference value for control, he torque of the engine, which is a physical quantity directly acting on the control of the vehicle.
A control system for a direct injection gasoline engine (arranged to inject fuel directly into the combustion chamber) is shown in Japanese Patent Publication No. 63-159614. This system is designed to produce extremely lean combustion at an air-fuel ratio of about 40 to 50 at a low speed, low load operating region in order to improve fuel consumption; and to enrich the air-fuel ratio continuously or in a stepwise manner as the load or speed increases. The set air-fuel ratio is not necessarily determined as a constant in accordance with an operating condition. For example, the air-fuel ratio can conceivably be set near the theoretical air-fuel ratio at the time of a cold engine operation, at which lean combustion of a stratified gas mixture is difficult.
In the case of an engine in which the air-fuel ratio is varied widely in this way, a direct relation between the generation torque and the quantity of the intake air is lost. In the case of control of the generation torque, it is necessary to control the intake air quantity in accordance with the set air-fuel ratio.
Namely, an appropriate method to control the generation torque of such an engine to control the vehicle motion, or the revolution speed at idle, is to first set a target value as an intermediate variable, such as a target generation torque, and then determine a manipulated variable(s) (intake air quantity and fuel injection quantity) to achieve the target, instead of directly controlling the air quantity, which has no direct relationship with the generation torque. Much attention has been given to an engine control apparatus employing engine torque demand control.
On the other hand, an engine control system can be arranged to selectively use utterly different control methods for idling operation and for non-idle operation. For example, engine torque demand control can be used for non-idle operation and some other control method can be used for idle operation. However, changeover between the control methods poses a difficult problem as to how to provide a smooth transition between the idle state and the non-idle state.
When a vehicle is in an idle running state and the generation torque in the idle state is relatively great, and for example the driver depresses the accelerator slightly, the control technique might be changed from idle control to non-idle control and a predetermined generation torque may be produced at that state. However, such a predetermined generation torque can be smaller than the generation torque dictated by the idle control technique due to the differences in the control techniques. In such a case, despite the slight depression of the accelerator, the vehicle speed decreases, contrary to the driver's intention, causing a very unnatural bodily feeling.
The inventors have recognized that it is desirable to configure an engine control system to employ the same basic control technique of torque demand control regardless of whether the vehicle is in the idle condition or the non-idle condition, to thereby improve driveability. However, as the inventors have also recognized, there are problems with employing torque demand control for both idling operations and non-idling operations.
In the above-mentioned Japanese Patent Publication No. 1-313636, the throttle opening degree (.theta..sub.o) to control the supply air quantity is set to a characteristic such as shown in FIG. 1 (which shows target torque To versus engine speed Ne). In FIG. 1, the characteristic is such that, if the target torque (To) is constant, the throttle opening degree is increased as the engine speed (Ne) increases. This means that in the state of the same throttle opening degree, the torque decreases as the engine speed increases. This is the same as the characteristics of an ordinary engine.
As an example of loss load torque, to be added to the required output torque, it is possible to identify internal losses of the engine such as engine friction and pumping loss. Characteristics, as shown by way of example in FIG. 2, indicate that the load torque decreases with a decrease in the engine speed in the region of normal engine speeds. FIG. 2 shows the friction loss due to piston(s) and cam(s), and the load of pumps such as the water pump and oil pump, together. Other loads also have approximately-similar tendencies. As a whole, the load torque generally decreases with a decrease in the engine speed.
Therefore, the control system in a torque demand system is fundamentally arranged to have characteristics like those in FIG. 1.
During driving, in response to the required torque, the engine torque demand system opens the throttle in accordance with the engine speed and increases the supply air quantity as the engine speed increases.
If the air-fuel ratio of the gas mixture formed in the combustion chamber is constant (for example, at the theoretical air-fuel ratio), the generation torque is approximately proportional to the mass of air sucked into the cylinder (the air mass per cylinder). Therefore, it is necessary to supply the intake air quantity (the quantity of flow per unit time) in proportion to the engine speed to produce the same torque irrespective of variation of the engine speed. Accordingly, air quantity manipulation of opening the throttle with an increase in the engine speed is proper.
However, in addition to the normal driving state, there is the idling state during which the engine speed is maintained at a low level to prevent the engine from stopping. In the idling state, there is the following problem when torque demand control for normal operation is applied to idling.
During an idle operation, if the load is increased by some disturbance (such as by shifting from neutral to drive, turning on the air conditioner, and/or turning on the rear defogger) and the engine speed decreases, torque demand control acts in the direction to close the throttle, even though the target torque remains constant, as evident from the characteristics of FIG. 1. That is, in spite of the revolution decrease and the need for an increase in the air quantity to increase the speed again, the system decreases the air quantity and acts contrary to demand, in the idle state.
Thus, as the inventors have recognized, when idling, if the load increases due to some disturbance, the engine speed decreases, and the conventional torque demand system decreases the target generation torque in accordance with the decreased engine speed. That is, restoration of engine speed is desired, but the conventional control works in a direction to decrease the generation torque, and restoration of the engine speed may not be accomplished.